Hybrid-secondary uncluttered permanent magnet machine and method

ABSTRACT

An electric machine ( 40 ) has a stator ( 43 ), a permanent magnet rotor ( 38 ) with permanent magnets ( 39 ) and a magnetic coupling uncluttered rotor ( 46 ) for inducing a slip energy current in secondary coils ( 47 ). A dc flux can be produced in the uncluttered rotor when the secondary coils are fed with dc currents. The magnetic coupling uncluttered rotor ( 46 ) has magnetic brushes (A, B, C, D) which couple flux in through the rotor ( 46 ) to the secondary coils ( 47   c   , 47   d ) without inducing a current in the rotor ( 46 ) and without coupling a stator rotational energy component to the secondary coils ( 47   c   , 47   d ). The machine can be operated as a motor or a generator in multi-phase or single-phase embodiments and is applicable to the hybrid electric vehicle. A method of providing a slip energy controller is also disclosed.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Contract No.DE-AC05-000R22725 awarded to UT-Battelle, LLC, by the U.S. Department ofEnergy. The Government has certain rights in this invention.

TECHNICAL FIELD

The field of the invention is brushless machines, including both ac anddc machines, including both motors and generators, and includinginduction machines, permanent magnet (PM) machines and switchedreluctance machines.

DESCRIPTION OF THE BACKGROUND ART

A brushless doubly-fed induction motor (BDFIM) has two sets of statorwindings for two p-poles and two q-poles. The rotor winding has a nestedcage with 2(p+q) poles. It produces a motor control with a relativelynarrow range of speed control. The existing extended rotor cagetechnology has a drawback in that the both rotational and slip energyare transferred in a cluttered approach to energy transfer.

An induction machine may be viewed as a transformer with its stator asthe primary and the rotor as the secondary. A slip-ring wound-rotorinduction motor, with a secondary winding is connected through a set ofslip rings and brushes, has been known for decades. By changing theresistance connected to the brushes, the starting current and the speedof the motor can be changed. However, maintenance of a motor with sliprings and brushes is expensive.

It is generally agreed that the most significant energy savings forelectric motor drives comes from the adjustable speed drive and that themotor plays a relatively less significant role. The high cost ofadjustable speed drives fed by an adjustable-frequency inverterdiscourages many potential users. There are many other known adjustablespeed methods. The brushless doubly-fed motor (BDFM) provides anadjustable-speed control having a lower initial cost than otheralternatives.

Hsu, U.S. Pat. No. 6,310,417, issued Oct. 30, 2001, disclosed ahybrid-secondary uncluttered induction machine that has a significantpotential to lower the cost of adjustable-speed drives. In addition tospeed control below synchronous speed, this machine may also be operatedabove synchronous speed.

The term “hybrid secondary” as it relates to such a machines impliesthat several secondary circuits can be used in various combinations fordifferent applications. Examples of such secondary circuits are avariable resistance circuit, an inverter circuit for doubly-fedoperation, and a generator circuit.

The term “uncluttered coupling” relates to a stator and rotor thatcouple slip energy. In an induction motor, the speed of the rotatingstator field equals the sum of 1) the speed of the rotating rotor fieldplus 2) the mechanical rotation speed of the rotor. With the motorrunning at maximum torque and close to synchronous speed, rotor speed ishigh and slip (the difference between the speed of the rotating statorfield and the rotational speed of the rotor) is small, about 3 to 10percent, and the slip frequency induced in the rotor is small, perhapstwo to six cycles per second for a 60 Hz motor.

To couple only slip energy, the stator and rotor have coils that runcircumferentially, sometimes referred to as “peripherally,” around theaxis of rotor rotation. The peripheral coils of the rotor and stator aremagnetically coupled. The rotor coil rotates and carries aslip-frequency current. Because the rotation does not change the totalmagnetic flux linking both the rotor and stator coils, no electromotiveforce (emf) is induced in the stator coil due to the rotation of therotor coil. This “uncluttered coupling” allows only the slip energypower corresponding to the slip-frequency currents to be transferredbetween the rotor and stator coils of the transformer.

It is desired to make such a machine that is more compact and has fewerparts while still providing a source of slip energy for speed control.

SUMMARY OF THE INVENTION

This invention provides a multiple-rotor permanent-magnet (PM) machinewith a rotor that couples a slip flux to one or more secondary coilsthrough a magnetic coupling uncluttered rotor. Consequently, for theapplications such as the hybrid electric vehicles (HEV) that require twoindividual machines, the size and cost saved by using a single machineis significant. The invention eliminates the main rotor and theauxiliary rotor with windings which were present and electricallyconnected in the prior art. The stator and slip energy rotor are nowmagnetically coupled and a slip energy source is provided by peripheralsecondary coils in which slip current is induced through the magneticcoupling uncluttered rotor without rotor windings.

The invention relates to an electric induction machine comprising astator having coils for receiving ac electrical power to provide amagnetic field; a permanent magnet rotor spaced from the stator todefine a first air gap relative to an axis of rotation for the permanentmagnet rotor; a magnetic coupling uncluttered rotor spaced from thepermanent magnet rotor to define a second air gap relative to an axis ofrotation for the permanent magnet rotor; and at least one stationarycore and secondary coil spaced from the magnetic coupling unclutteredrotor by a third air gap. The magnetic coupling uncluttered rotor isprovided with a plurality of magnetic elements for coupling flux to thesecondary coil, and the secondary coil is disposed around an axis ofrotation for the rotor to allow induction of a slip energy current inthe coil without inducing a rotational energy current.

In the new approach, the PM rotor, the magnetic coupling unclutteredrotor, and the secondary coil(s) form an uncluttered slip energy machineexcept the PM rotor is acting as an armature that produces either arotating or a standstill flux wave between the PM rotor and theuncluttered rotor. For example, when the PM rotor is at standstill, thecurrents in the secondary coil(s) can produce a torque between the PMrotor and the magnetic coupling uncluttered rotor. Under a relativerotation between the magnetic coupling uncluttered rotor and the PMrotor, the secondary coil(s) can act as either a generator or a motordepending on the direction of current in the coils.

The new approach does not require a rotor winding in either rotor toproduce the slip-frequency currents that are subsequently fed to thecoil(s) of a peripheral transformer. The function of the new rotors areto directly transform the rotating air-gap flux originated by the PMrotor to the uncluttered flux seen by the stationary secondary coils.

The invention is also practiced in a method of providing A method ofproviding slip energy control in an electrical machine, the methodcomprising inducing a flux in a magnetic coupling uncluttered rotoracross an air gap by conducting a current in a primary winding of thestator and by positioning a permanent magnet rotor with permanentmagnets in said air gap; positioning a secondary coil across a secondair gap from the rotor; and inducing a slip current in the secondarycoil by magnetically coupling the flux through the magnetic couplinguncluttered rotor without inducing a current in the magnetic couplinguncluttered rotor.

Other objects and advantages of the invention, besides those discussedabove, will be apparent to those of ordinary skill in the art from thedescription of the preferred embodiments which follows. In thedescription reference is made to the accompanying drawings, which form apart hereof, and which illustrate examples of the invention. Suchexamples, however are not exhaustive of the various embodiments of theinvention, and therefore reference is made to the claims which followthe description for determining the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a hybrid-secondary uncluttered machine ofthe prior art;

FIG. 2 is a schematic view illustrating the operation of the machine ofFIG. 1;

FIG. 3 is a multi-graph of current and torque for high and lowresistance for pump and fan loads;

FIG. 4 is a perspective view of an inside of a dual stator in ahybrid-secondary uncluttered machine of the prior art;

FIG. 5 is a perspective view of a dual rotor in hybrid-secondaryuncluttered machine of the prior art;

FIG. 6 is an electrical schematic view of a hybrid-secondary unclutteredmachine of the prior art showing the parts that are made unnecessary bythe present invention;

FIG. 7 is a schematic diagram of a two-phase hybrid-secondaryuncluttered permanent magnet machine of the present invention;

FIG. 8 a is a sectional view taken in a plane indicated by line 8 a-8 ain FIG. 8 b;

FIG. 8 b is a plan view of a magnetic coupling, uncluttered rotor in thetwo-phase machine seen in FIG. 7;

FIG. 8 c is a detail view of the magnetic brushes included in the rotorof FIGS. 8 a and 8 b;

FIG. 9 a is a sectional view taken in a plane indicated by line 9 a-9 ain FIG. 9 b;

FIG. 9 b is a plan view of a permanent magnet rotor in the two-phasemachine seen in FIG. 7;

FIGS. 10 a and 10 b are sectional and sectional views of a rotor for asingle-phase machine of the present invention;

FIG. 10 c is a detail view of the magnetic brushes included in the rotorof FIGS. 10 a and 10 b;

FIG. 11 is a sectional view of the machine of FIG. 7 including parts ofa hybrid electric vehicle interface; and

FIG. 12 is a schematic view of the machine of FIG. 7 applied to a hybridelectric vehicle application.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an assembly 10 of an induction motor 11 of the priorart that includes an uncluttered transformer 12 for providing a hybridsecondary. A stator core 13 of the motor 11 is wound with a polyphasewinding 14. A rotor core 15 of the motor is wound with a two-phasewinding 16. One or more conductors of cast aluminum can also be used onthe rotor 15. To the left is a two-phase uncluttered rotatingtransformer 12. The stator 17 and stator coils 18 of this transformer donot connect to the stator winding 14 of the motor or to the rotor 19 ofthe transformer 12, but instead are magnetically coupled to the rotor 19of the transformer 12. The stator 17 and the rotor 19 of the transformer12 have coils 18, 20 that are peripherally disposed around the axis ofrotation 21 for the motor 11 and the transformer 12. The electricalconnection of the machine rotor 15 to the transformer rotor 19 allowsonly the slip energy to be present in the rotor 19 of the transformer12, and this slip energy is coupled to the stator 17 of the transformer12, which has windings for supplying this energy to another device.

FIG. 2 shows a schematic of the peripherally wound stator coil 18 andthe peripherally wound rotor coil 20 that carries a slip energy current.Magnetic flux 23 is coupled between the stator coil 18 and the rotorcoil 20. The rotor 19 rotates with the motor shaft 22 but the stator 17is stationary. The rotation of the rotor 19 does not induce anyadditional energy beyond the slip energy in the rotor 19, so only theslip energy is coupled to the stator coil 17 of the transformer 12 inFIG. 1.

FIGS. 4 and 5 show a physical example of the peripherally disposedcoils. FIG. 4 shows a machine housing 30 with the rotor removed toexpose the motor stator 31 and the transformer stator 32. Thetransformer stator 32 exhibits the peripherally disposed coils 33 inFIG. 4. FIG. 5 shows the induction motor rotor 34 with rotor teeth 35and the transformer rotor 36 with peripherally disposed coils 37.

The hybrid secondary uncluttered induction machine portion used in thepresent invention is advantageous for controlling fan and pump loads, aswell as being suitable to be incorporated into the drive train ofelectric or hybrid electric vehicles. Fan and pump loads represent twothirds of the motor drives in industry. The required adjustable speedrange and the load torque versus speed curve as showed in FIG. 3,dictate the rating of the slip power controller of an induction motor.

For a fan load without or with a backpressure, the required fan powermay be proportional to the cube or to the square of speed, respectively.Assuming unity efficiency and power factor, the per-unit slip power forthese two examples may be roughly estimated as.(per unit slip power)≈slip·(1−slip) ^(3 or 2)  (1) TABLE 1 Example ofper-unit slip powers of fan loads Based on stator Per unit slip powerrotating field W/o back pressure With back pressure slip slip(1 − slip)³slip (1 − slip)² −0.2 −0.346 −0.288 −0.1 −0.133 −0.121 0 0 0 0.01 0.00970.0098 0.1 0.0729 0.081 0.3 0.1029 0.147 0.5 0.0625 0.125 0.7 0.01890.063 0.9 0.0009 0.009 1.0 0 0

Table 1 shows that the rating of the positive slip power of a fan loadis generally low for speeds below synchronism. Subsequently, even withconsideration of the non-unity of power factor and efficiency, therequired rating of the control for speed adjustment is low. This enablesthe use of a very small power electronics module or small adjustableresistors to control a motor with a high power rating. These energyabsorption approaches can be characterized as an effective resistanceapproach.

In order to control the rotor slip energy of an induction motor that hasno slip rings, the rotor current must be coupled to a stationary controlcircuit through a rotating transformer of the type illustrated in FIG.1.

FIG. 6 schematically illustrates a difference between the hybridsecondary uncluttered induction machine portion used in the presentinvention and the hybrid secondary uncluttered machine of the prior art.In this machine, the rotor 15 of the induction machine 11 and the rotor19 of the transformer 12 and their electrical interconnection areomitted as represented by the “X's” and a single rotor 46 formagnetically coupling energy between the machine stator and the statorcoils of the transformer is added as seen in FIG. 7.

FIG. 7 shows an example of an axial gap embodiment of the presentinvention. The machine 40 includes a housing 41 and a first rotor shaft42 mounted on bearings 45 for rotation of a first, uncluttered rotor 46in the housing 41. A second rotor 38 carrying permanent magnets 39 ispositioned between a stator 43 and the first rotor 46. The second rotor38 is mounted for rotation on a hollow shaft 28 supported by a secondset of bearings 29. The stator 43 receives multi-phase electric powerthrough lines 44 connecting to multi-turn windings. A toriodal secondarycore and coil assembly 47 has peripherally disposed windings whichencircle the first rotor shaft 42. A first axial air gap relative to therotor shafts, 28, 42 is located between the stator 43 and the PM rotor38. A second axial air gap 27 is provided between the PM rotor 38 andthe uncluttered rotor 46. And, a third axial air gap 49 is providedbetween the first rotor 46 and the secondary core and coil assembly 47.The uncluttered rotor 46 rotates with the shaft 42 and has non-contactmagnetic brushes (not shown in FIG. 7) for conducting flux.

In this invention, the uncluttered rotor 46 is magnetically coupled tothe PM rotor 38 instead of the armature 43. The machine 40 sees the PMrotor 38 as the rotating field. The PM rotor 38 naturally produces aflux wave that is either stationary or rotating.

With reference to FIGS. 8 a and 8 b, an n-phase flux path for therotating air-gap flux is formed on one side of the uncluttered rotor 46facing the PM rotor 38. On the other side of the uncluttered rotor 46,the 2*n non-continuous rings are formed by the step-up portions 50 ofthe magnetic brushes A, B, C and D. The n-phase secondary toroidal coresand coils 47 are coupled with the non-continuous rings for linking withthe uncluttered fluxes that do not contain the rotation-frequency fluxcomponent.

The n-phase secondary toroidal coils can be connected to differentcomponents such as the variable resistors or an inverter for speedcontrols in a motor mode, or to the electric loads in a generator mode.The uncluttered rotor 46 and the secondary toroidal cores and coils 47are all parts of the secondary circuit. They are in the magnetic path ofthe permanent magnets 39 for controlling the air-gap flux densitybetween the stator 43 and the PM rotor 38 for the field weakening andfield enhancement modes, respectively. The PM rotor 38 can be operatedin a motor mode or a generator mode depending on the currents fed to thestator 43.

The PM rotor 38, the uncluttered rotor 46, and the secondary toroidalcores and coils 47 form an uncluttered slip energy machine except the PMrotor 38 is acting as an armature that produces either a rotating or astandstill flux wave between the PM rotor 38 and the uncluttered rotor46. For example, when the PM rotor 38 is standstill, the currents in thesecondary toroidal coils 47 can produce a torque between the PM rotor 38and the uncluttered rotor 46. Under a relative rotation between theuncluttered rotor 46 and the PM rotor 38, the secondary toroidal coils47 can act as either a generator or a motor depending on the directionof current in the coils 47.

FIGS. 8 a, 8 b and 8 c show the details of the first rotor 46 withmagnetic brushes A, B, C and D for a 2-phase, eighteen pole device.Using symbol, n, as the number of phases of the uncluttered rotor 46,each pole pair area (i.e., two times the pole-pitch 51) of the rotorconsists of 2*n (i.e., 4) groups of magnetic brushes A, B, C and D. Twoinner groups C, D of these four magnetic-brush groups A, B, C and D formone phase, and two outer groups A, B, form another phase. The detailviews of these four groups of flux brushes, A, B, C, and D, are shown inFIG. 8 c.

The magnetic brushes A, B, C and D can be made of stacked laminations,compressed powder cores, ferromagnetic wires or other equivalent softmagnetic materials that have good magnetic permeability, a highsaturation level, and low core-loss properties. The magnetic brushes A,B, C and D are secured between the non-magnetic outer ring 54 and thenon-magnetic rotor hub 55. A two-phase flux path for the rotatingair-gap flux is formed on one side of the rotor 46 facing the armature43. On the other side of the rotor 46 are the step-up portions 50 (FIGS.8 a and 8 b) of the magnetic brushes A, B, C and D that form fournon-continuous rings. The rings are separated in a radial direction byring-shaped gaps 58 (FIGS. 8 a and 8 b), which are made of materialbetween each pair of magnetic brushes in each phase, each magnetic brushin the pair being separated on the secondary side from its counterpartmagnetic brush by a ring-shaped air gap 59 (FIGS. 8 a and 8 b). Therings of magnetic brushes are interrupted by radial gaps 56 (FIG. 8 b)between the magnetic-brush groups can be filled with non-magneticmaterials. Because the summation of the opposite-polarity fluxes passingthrough the magnetic brushes per pole pair is zero, the boundary spaceof every pole pair can be made of electrically-conducting non-magneticmaterials. This allows the rotor 46 to have sufficiently high mechanicalstrength required by certain designs. The outer ring 54 should bedesigned to withstand the centrifugal force of the rotor 46.

FIGS. 9 a-9 b show the permanent magnet (PM) rotor 38 with alternatingnorth (N) and south (S) pole permanent magnets 39, one pair per apole-pair pitch 73. The magnets are also oriented N-S or the reversethrough the thickness of the PM rotor 38 as seen in FIG. 9 a. The rotorhas an inner ring 70 and an outer ring 71 and radial gap portions 72formed of non-magnetic material.

FIGS. 10 a-10 c show a single-phase (i.e., n=1) uncluttered rotor 60that can be used as a generator and a motor but without starting torqueat standstill when it is fed by the secondary toroidal coils. The rotor60 has pole pitch distance 61, just one ring of magnetic brushes 62 (A&Bseparated by space 67), an outer ring of non-magnetic material 64, aninner hub of non-magnetic material 65 and non-magnetic radial, spacedportions 66 between pole pairs forming an 18-pole rotor 60.

When the uncluttered rotor is constructed in a single-phase (i.e., n=1),the machine can be used as a generator and motor but without startingtorque at standstill. The machine would produce starting torque when theuncluttered rotor is constructed for two or a higher number of phases.

FIG. 11 shows the details of the machine 40 of FIG. 7 applied to ahybrid electric vehicle application. The stator 43 more particularlyincludes a stator core 43 a and a multi-phase winding 43 b having aplurality of turns or coils. Two secondary toroidal cores 47 a, 47 b andthe corresponding peripherally disposed coils 47 c, 47 d are shown forcoupling with the four non-continuous rings of magnetic brushes A, B, Cand D to link with the uncluttered fluxes that do not contain therotating-frequency flux component.

A planetary gearset 74 couples an engine shaft 76 in a vehicle to themotor shaft 42 through a rotational bearing assembly 80, 81. At leastone of the gears 75 would be coupled to drive the wheels of the vehicle.The hollow shaft 28 and the bearings 29 for the PM rotor 38 are seen inmore detail.

Various embodiments can be constructed according to the presentinvention provided that an n-phase flux path for the rotating air-gapflux is formed on one side of the uncluttered rotor 46 facing thearmature 43. On the other side of the rotor, the 2*n non-continuousrings are formed by the step-up portions 50 of the magnetic brushes. Then-phase secondary toroidal cores 47 a, 47 b and coils 47 c, 47 d in FIG.9 are coupled with the non-continuous rings for linking with theuncluttered fluxes that do not contain the rotation-frequency fluxcomponent.

The n-phase secondary toroidal coils 47 c, 47 d can be connected todifferent components such as the variable resistors or an inverter forspeed control in a motor mode, or to the electric loads in a generatormode.

FIG. 12 shows the complete vehicle application with vehicle engine 92coupled to the PM rotor 38 through the hollow drive shaft 28 andplanetary gearing 74 and coupled to the uncluttered rotor 46 through theplanetary gearing 74 and drive shaft 42. Also shown is the coupling ofthe drive mechanisms to the wheel 94 through gears 75 in the planetarygearing arrangement 74. The gearing applicable to the present inventionis not limited to planetary gearing; a differential gearset with onlythree shafts can also be employed. A battery 90 is connected to aninverter/rectifier 91 to supply power and to be charged. Theinverter/rectifier 91 controls the form of the power supplied to thestator 43 a, 43 b.

The invention provides a method that simplifies the earlier unclutteredinduction machine by transforming the conventional rotating flux in theair gap facing the armature to an uncluttered flux (i.e., without therotation-frequency flux component) facing the secondary toroidal coresand coils.

The uncluttered machine of the prior art cannot operate at synchronousspeed because there is no induced current in the rotor at synchronousspeed. The machine of the present invention can operate at synchronousspeed, because a flux that includes a dc flux can be passed through therotor at synchronous speed.

Suitable magnetic brushes can be made of materials with goodpermeability, high magnetic saturation level, and low core loss. Stacksof thin laminations of flux conducting materials, bundles offerromagnetic wires, or low ac loss compressed powders are materialexamples for the magnetic brushes and for the secondary toroidal cores.

The machine of the present invention is ideal for the hybrid electricvehicle application, but is not limited to this application.

The invention can be used in both axial-gap and radial gap machines.

A radial gap, high-strength undiffused machine can also be provided withthe present invention. The primary air gap between the rotors and thestator would be disposed a radial distance from the axis of rotorrotation. One or more secondary coil assemblies would be provided at theend of the cylindrical rotor(s) in a radial gap machine. The magneticcoupling uncluttered rotor would then be provided with magnetic brushesin a suitable pattern in place of the familiar conduction bars of aninduction motor. For an example of the general machine configuration anda detailed description, reference is made to Hsu, U.S. patentapplication Ser. No. 10/668,586, filed Sep. 23, 2003.

This has been a description of the preferred embodiments of theinvention. The present invention is intended to encompass additionalembodiments including modifications to the details described above whichwould nevertheless come within the scope of the following claims.

1. A hybrid-secondary uncluttered permanent magnet machine, comprising:a stator having coils for receiving ac electrical power to provide amagnetic field; a permanent magnet rotor spaced from the stator todefine a first air gap relative to an axis of rotation for the permanentmagnet rotor; a magnetic coupling uncluttered rotor spaced from thepermanent magnet rotor to define a second air gap relative to an axis ofrotation for the permanent magnet rotor; at least one stationary coreand secondary coil spaced from the magnetic coupling uncluttered rotorby a third air gap; wherein the magnetic coupling uncluttered rotor isprovided with a plurality of magnetic elements for coupling flux to thesecondary coil; and wherein the secondary coil is disposed around anaxis of rotation for the rotor to allow induction of a slip energycurrent in the coil without inducing a rotational energy current.
 2. Themachine of claim 1, wherein the first air gap is an axial air gapdisposed along the axis of rotation of the rotor, wherein the second airgap is an axial air gap disposed along the axis of rotation of therotor, and wherein the third air gap is an axial air gap disposed alongthe axis of rotation of the rotor.
 3. The machine of claim 1, whereinthe magnetic elements are provided in pairs for each electrical phase ofpower supplied to the stator coils.
 4. The machine of claim 2, whereineach pair of magnetic elements is spaced apart in a radial direction bya non-magnetic ring on a side of the rotor facing the least onestationary, secondary core and coil.
 5. The machine of claim 1, whereinthe magnetic elements are magnetic brushes made of stacked metallaminations.
 6. The machine of claim 1, wherein the magnetic elementsare magnetic brushes made of a compressed powder material havingferromagnetic properties.
 7. The machine of claim 1 wherein the magneticelements are magnetic brushes made of ferromagnetic wires.
 8. Themachine of claim 1 wherein the magnetic coupling uncluttered rotor ismade of electrically conducting non-magnetic material in portions whichsupport and separate the magnetic elements.
 9. The machine of claim 1,wherein the permanent magnet rotor has permanent magnets positioned toprovide alternating polarity, said permanent magnets being aligned alongradii of the permanent magnet rotor.
 10. The machine of claim 9, whereinthe said permanent magnets are in a round shape.
 11. The machine ofclaim 1, wherein the machine is an ac induction machine.
 12. The machineof claim 1, wherein the machine is a motor.
 13. The machine of claim 12,wherein the machine is a synchronous motor.
 14. The machine of claim 12,wherein the machine is a brushless dc motor.
 15. The machine of claim 1,wherein the machine is a generator.
 16. The machine of claim 1, whereinthere are a plurality of stationary, secondary cores with secondarycoils spaced from the rotor by the secondary air gap and disposed aroundthe axis of rotation for the rotor.
 17. The machine of claim 16, whereinthe secondary coils are adapted to be connected to supply slip energy toa resistive load.
 18. A method of providing slip energy control in anelectrical machine, the method comprising: inducing a flux in a magneticcoupling uncluttered rotor across an air gap by conducting a current ina primary winding of the stator and by positioning a permanent magnetrotor with permanent magnets in said air gap; positioning a secondarycoil across a second air gap from the rotor; and inducing a slip currentin the secondary coil by magnetically coupling the flux through therotor without inducing a current in the rotor.
 19. The method of claim18, further comprising supplying a dc current in the secondary coil toproduce a dc flux in the uncluttered rotor.
 20. The method of claim 18,wherein the primary air gap is disposed axially along an axis ofrotation for the rotor and wherein the second air gap is also disposedaxially along an axis of rotation for the rotor.
 21. The method of claim18, wherein the machine is operated as an ac induction machine.
 22. Themethod of claim 18, wherein the machine is operated as a motor.
 23. Themethod of claim 18, wherein the machine is operated as a generator. 24.The method of claim 18, wherein a dc excitation is provided to thesecondary coil for operating the machine at synchronous speed.
 25. Themethod of claim 18, wherein a dc excitation is provided to the secondarycoil for operating the machine as a brushless dc machine.